"Acting is like painting pictures on bathroom tissues. Ten minutes later you throw them away and they're gone." - Shelley Winters
Well, heaven knows she's right, as any actor will tell you. The same goes for musical performances, of course. Even when the performance is captured electronically, it's still just a representation of what that performance looked like at that point in time. On a different day it would have been different, even if only very subtly.
We performers are ultimately just writing love letters in the sand.